End corona shielding of the mentioned type is known from EP 2 362 399 A1, for example.
In the case of electrical operating means with a high electrical rated voltage, in particular a rated voltage of above 5 kV, live conductor bars, for example, need to have electrical insulation which is shielded from cavities and detachments by an inner and an outer conducting layer. Examples of such electrical operating means are electric generators, electric motors, transformers, bushings and electrical cables.
The conducting layers mean that an irregular distribution of the electrical field strength within the main insulation and thus partial discharges which can destroy the main insulation are avoided. The inner conducting layer located between the main insulation and the conductor bar is also referred to as inner potential grading (IPG). The outer conducting layer located on an outer side of the main insulation is also referred to as outer corona shielding (OCS). The electrical field emanating from the electrical conductor bar is reduced in the main insulation starting from the IPG in the radial direction up to the OCS.
In order to be able to connect a conductor bar of an operating means to a further electrical arrangement, the insulation needs to be stripped from one bar end. In other words, it is not possible to control the electrical field by means of the IPG, the main insulation and the OCS over its profile in a desired manner at one bar end. The problems occurring in this connection will be explained in more detail below with reference to FIGS. 1A-1C. FIG. 1A shows a bar end 10 of a generator winding bar 12 in the region of an exit point 14 of the winding bar 12 from a stator laminate stack 16 of an electrical generator for a high voltage. Outer corona shielding OCS which is electrically connected to a ground potential 18 ends at the exit point 14. Main insulation 20 is continued in the direction of a region 22 of the winding bar 12, in which region the insulation has been stripped.
This arrangement shown in FIG. 1A represents a typical sliding arrangement which can be provided not only in the case of generators, but also in electric motors, transformers, bushings and cables. FIG. 1B shows, in relation to the arrangement of FIG. 1A, equipotential lines 24 of an electrical field and lines of force 26 running perpendicular thereto. The electric field surrounds the live winding bar 12. In the case of the electrical field, the lines of force also have a strong nonlinear tangential component 30, in each case in addition to a radial component 28, in the exit region 14 at the end of the outer corona shielding OCS. Owing to a very dense profile of the equipotential lines 24 at one edge 32 of the outer corona shielding OCS, a magnitude of the electrical field strength is at its greatest there. A very low corona inception voltage results here. Therefore, undesired creeping discharges occur again and again in the region of the edge 32.
FIG. 1C illustrates a profile 37 of an electrical potential as results on an outer surface 34 of the main insulation 20 along a longitudinal direction of extent 36 of the winding bar 12.
In the region of the edge 32, the potential increases from ground potential (0% of the potential of the winding bar 12) in a region of a few millimeters to 100% of the potential of the winding bar 12. In other words, over this comparatively small path region, there is a voltage drop of 5 kV or more.
In order to prevent partial discharges at the edge 32, further outer corona shielding can be provided which surrounds the main insulation 20 on the other side of the exit point 14 as well. This outer corona shielding is then referred to as end corona shielding or cable end seal. Such end corona shielding comprises a resistive potential grading or field control, i.e. end corona shielding has a higher electrical resistance than an OCS. End corona shielding can be produced by a partially conductive enamel or a partially conductive banding on the basis of silicon carbide or another electrically semiconductive filler. Partial conductivity is in this case understood to mean a conductivity which is less than that of a metallic conductor and greater than that of an electrical insulator. The aim of such potential grading is to obtain a less steep profile of the surface potential of the main insulation 20.
Until now, it has been known in this regard to surround the surface 34 of a main insulator 20 beyond the edge 32 of the outer corona shielding OCS with a semiconductive enamel or a semiconductive banding (W. P. Schmidt et al.: “Umweltverträgliche Harzimprägnierung elektrischer Maschinen mittels Stromwärme” [Environmentally sound resin impregnation of electric machines by means of Joulean heat], German Federal Ministry for Transport, Innovation and Technology, reports from energy and environmental research, 62/2006). However, it is necessary that a resistance of the end corona shielding surrounding the main insulation 20 increases from the edge 32 to the region 22, from which the insulation has been stripped, of the winding bar 12. Otherwise, an arc can form between the end corona shielding and the region 22. In order to predetermine the electrical resistance of the end corona shielding along the direction of extent 36, a partially conductive enamel can be applied in a plurality of layers. The layers in this case extend over different distances along the direction of extent 36. The layer arrangement thus produced therefore has a variable thickness along the direction of extent 36. Correspondingly, provision can be made for the end corona shielding to be provided by a plurality of partially conductive bandings and in the process for the bandings to be wound more thickly in the region of the edge 32 than close to the region 22 from which the insulation has been stripped. The locationally dependent resistance is in both solutions achieved, therefore, by a correspondingly predetermined line cross section of the enamel layers or the bandings.
One disadvantage with these solutions is that transition points result in the region of the end corona shielding, i.e. either transitions between the enamel layers or between individual plies of the bandings. Then, an excessive increase in the field strength can occur here, as a result of which partial discharges can arise within the end corona shielding as well.